1. Field of the Invention
The present invention is related to a fault diagnostic system for vehicles which is adapted to communicate with an electronic control unit carried on a vehicle such as a car and provided with a self-diagnostic function of detecting and recording faults of various sensors and actuators, and based on the communication result, finds the failures of the electronic control unit and the peripheral equipment connected thereto.
2. Description of the Prior Art
Recently, a computer-aided electronic control unit (hereinafter referred to as ECU) has been increasingly mounted as a control unit such as an electronic fuel injection unit or anti-lock braking system (ABS). The ECU takes in the output signals from various sensors such as a pressure sensor for detecting the negative pressure of the air intake manifold, a temperature sensor for detecting the temperature of the cooling water for the engine, and a revolution sensor for detecting the number of revolutions of the engine, and controls the actuators according to predetermined programs based on these signals.
There is a known fault diagnostic system which is adapted to be connected to the ECU to check faulty parts of a vehicle when a failure occurs in the ECU or its peripheral equipment (sensors, actuators and connectors). A fault diagnostic program for finding out a faulty part is registered in the fault diagnostic system. The fault diagnostic system communicates with the ECU according to the fault diagnostic program stored therein to determine the faulty part based on the communication result, and displays it on a display device (LCD or CRT). In addition, if it is judged that there are a plurality of faulty parts, all of them are displayed on the display device. The repairman can confirm the faulty parts by this display and promptly take a proper action.
Since a plurality of fault diagnostic programs are prepared in general, a fault diagnostic program suitable for a target ECU to be tested is manually or automatically selected prior to fault diagnosis.
In the system in which a fault diagnostic program is automatically selected, the identification code of the ECU (hereinafter, referred to as ECU-ID) connected to the system is first obtained from the ECU. According to the ECU-ID thus obtained, a predetermined diagnostic program is selected from the plurality of faulty diagnostic programs prepared in advance.
An example of the fault diagnostic system adapted to be connected to the ECU carried on a car through a bidirectional communication interface is described in the Japanese Patent Kokai official gazettes 64-52551 and 63-78041.
In the conventional fault diagnostic system, a fault diagnostic program is selected only based on a type of an ECU. On the other hand, if a faulty vehicle is repaired, the fault diagnostic program and the vehicle must conform to each other.
However, the vehicle or engine type may differ even for the same ECU, and thus it may take time to identify the faulty portion and/or part if a fault diagnostic program is selected only based on the ECU type as has conventionally been done. As a result, instructions on the parts supply based on the diagnostic result may not be properly provided.
Further, there is a system which transmits the diagnostic result to the maker's host computer through a public telecommunication line. By such system, the car maker can classify and store the transmitted data to classify and analyze the tendencies and causes of failures, and establish a proper and prompt quality certification and parts supply systems. However, with the traditional method of storing failure data only by classification for each ECU, it is difficult to accurately recognize the causes and tendencies of failures.
In the conventional system, if a plurality of faulty portions are detected, all of them are displayed, but it may be possible that some of them are not really out of order. For instance, since a particular control system affected by the error signal output from the true faulty portion may output a signal deviating from a reference value for normal operation, said particular control system also would be judged faulty. An example of such a case, wherein an apparent faulty state of one part is produced by the true failure of a different part, is described with reference to FIG. 6.
In FIG. 6, for instance, water temperature sensor diagnostic means 31A monitors the output of water temperature sensor 34A, and generates a water temperature fault signal if the output signal deviates from a predetermined characteristic or threshold value, and displays the fault thereof on a display device. Air-fuel ratio control system diagnostic means 31B monitors the solenoid activation signal of injector (fuel injection unit) 35A, the output signal of O.sub.2 sensor 36A and the operation of air-fuel ratio control system 37A, and causes a fuel system fault code to be displayed on the display device if there is any failure.
In the above fault diagnostic system, if water temperature sensor 34A is actually faulty and wrong water temperature data is supplied to air-fuel ratio control system 37A, air-fuel ratio control system 37A performs a predetermined processing on the basis of the wrong water temperature data and outputs a wrong activation signal to injector 35A. Since this activation signal is based on the wrong water temperature data, an adequate fuel injection is not performed, and eventually the output signal of O.sub.2 sensor 36A deviates from a predetermined range. As a result, a fuel system fault signal is output from air-fuel ratio control system diagnostic means 31b. As is obvious, since O.sub.2 sensor 36A is not actually faulty, the faulty state is still unimproved after O.sup.2 sensor 36A is repaired or replaced according to the display of the fuel system fault signal.
In the prior art, the true faulty portion is estimated from a plurality of fault displays by the experiences and intuition of an operator of diagnosis to determine priority, and the faulty portion is further diagnosed and repaired based on the estimated priority. However, if the estimation of the operator happens to be wrong, a portion other than the true faulty portion is subjected to repairs, and thus there is a problem that the repairs are time-consuming because useless check work is included until the repairs are completed.
Strictly speaking, the fault detection of sensors and the like by an electronic control unit as described above is the detection of fault between the microcomputer in the electronic control unit and the sensor lines including wire harness and connector, and thus even if the electronic control unit detects the failure of a sensor, it cannot judge whether the failure is continuous or transient (temporary). Here, "continuous fault" means a fault which continues once it has occurred, such as a failure of the sensor itself or breaking of wire harness, while "transient (or temporary) fault" means a fault which is not always continued, such as a contact failure of the connector for connecting the sensor and the wire harness or connecting the wire harness and the electronic control unit, that is, the contact failure may or may not occur due to vibration or the like during the running of the vehicle.
For this, even after the fault of a sensor or wire breaking of a wire harness (continuous fault) has been detected by the electronic control unit, it is required to further check for the contact failure of the connector (transient fault) using other proper means, and therefore, fault diagnosis of the vehicle is difficult and the procedure thereof is complex, too.
As described above, when the ECU has a self-diagnostic function, if an abnormal signal out of a reference range is detected at the input terminals to which various connectors are connected, the ECU judges that a fault has occurred, and stores a code (fault code) for identifying the portion which has generated the abnormal signal and a value of the abnormal signal (fault data) (in this specification, hereinafter these may be expressed, in combination, as fault information).
The fault diagnostic system stores a fault diagnostic program, communicates with the ECU according to the fault diagnostic program, determines the faulty portion from the communication result (the above-mentioned fault code and fault data), and displays the faulty portion on a display device (LCD). The repairman verifies the faulty portion on the basis of the display and takes an appropriate action. Even if the failure of a sensor is detected by the ECU, the faulty portion is not always the sensor itself, but it may possibly be the microcomputer itself in the ECU or only a portion on the sensor line including the wire harness and connector.
Accordingly, the ECU cannot accurately identify the specific faulty portion even if it has detected the fault of a sensor. In order to finally identify the faulty portion, the repairman must refer to the repair manual or the like to examine the connector number, the pin number of the connector, the wire color, etc. constituting the wiring system, and must check the wire harness and connector using an inspection apparatus such as a tester. In addition, since the fault judgment as to whether or not it is faulty must be performed by the repairman according to the indicated value of the tester, the repairman must be skilled.
Moreover, there is a problem that fault diagnosis is difficult for faults in which the fault degree is difficult to quantitatively recognize, or faults in which the faulty portion cannot be identified from only the fault degree even if it can be quantitatively grasped.
Faults which cannot be diagnosed by the conventional fault diagnostic system because of inability to quantitatively recognize the fault state or faulty portion, are described below by taking, as examples,
(1) deterioration of the "O.sub.2 sensor" for detecting the air-fuel ratio (air amount/fuel amount) based on components of the exhaust gas, PA1 (2) failure of "EACV" (Electrical Air Control Valve) for providing a bypass between the upstream and downstream sides of the throttle valve, and PA1 (3) failure of "EGR" for recirculating the exhaust gas into the combustion chamber of the engine.
FIG. 31 is a block diagram of the suction and exhaust system of an engine for explaining the functions of the O.sub.2 sensor, EACV and EGR.
To engine 173 are connected inlet manifold 175 for supplying a fuel gas and outlet manifold 174 for exhausting the gas after combustion. Throttle valve 176 is built in inlet manifold 175, and the throttle valve 176 is opened or closed by operation of the accelerator to control the number of revolutions of the engine.
Connected to inlet manifold 175 is EACV 170 for bypassing between the upstream and downstream sides of throttle valve 176. EACV 170 is an electromagnetic valve for supplying the fuel gas in surplus to engine 173 to increase the number of revolutions of the engine when a heavy lead is applied to the engine, as in the start-up or warm up of the engine, or when an electric lead such as an air conditioner is applied. EACV 170 consists of bypass passage 177 for bypassing the fuel gas, bypass valve 178 for adjusting the flow rate of the fuel gas, and solenoid coil 179 for controlling the opening of bypass valve 178.
The flow rate of the fuel gas bypassed by EACV 170 is continuously controlled by varying the magnitude of the electric current supplied from EACV driver circuit 172 to the solenoid coil 179 according to the instructions from ECU 1.
On the other hand, outlet manifold 174 and inlet manifold 175a downstream of throttle valve 176 are connected via EGR 150. EGR 150 is an exhaust gas recirculation equipment which recirculates the exhaust gas into the combustion chamber of the engine for afterburning of the unburnt gas, thereby to reduce the generation of NOx, and consists of first body 152 having control port 151, and second body 156 having atmospheric port 153, intake port 154, and exhaust port 155.
First body 152 and second body 156 are partitioned by diaphragm 157. Provided in the center of diaphragm 157 are compression spring 158 pressing diaphragm 157 toward second body 156, and valve plug 159 having one end fixed to diaphragm 157 and the other end opposed to exhaust port 155.
Displacement of valve plug 159 is detected by lift sensor 161. In second body 156, partition 160 is provided for separating the inside thereof between atmospheric port 153 and intake port 154. The output of lift sensor 161 is connected to ECU 1. Control port 151 is connected to inlet manifold 175 via electromagnetic valve 182 whose valve opening is controlled by ECU 1.
Since the recirculation amount of the exhaust gas (EGR amount) by EGR 150 depends on the opening of or position of valve plug 159, ECU 1 calculates the EGR amount; on the basis of the position signal of valve plug 159 given by lift sensor 161, and increases the valve opening of electromagnetic valve 182 if the EGR amount is less than a predetermined amount. As a result, the negative pressure of control port 151 becomes higher and diaphragm 157 is attracted toward first body 152 against the repulsion force of compression coil spring 158, so that the EGR amount flowing from intake port 154 to exhaust port 155 increases.
Provided downstream of outlet manifold 174 is O.sub.2 sensor 183, the output signal of which is input to ECU 1. ECU 1 calculates the air-fuel ratio based on the detection signal provided by O.sub.2 sensor 183, and controls the fuel amount injected by an injector (not shown) so that an optimum air-fuel ratio is obtained. The output of cooling water sensor 187 is provided to ECU 1, too.
Now, the causes of faults in the various portions shown in FIG. 31 are described.
(1) Deterioration of O.sub.2 Sensor
The relationship between the air-fuel ratio in the supplied fuel gas and the output voltage of the O.sub.2 sensor is as shown in FIG. 32. As it is known that the ideal air-fuel ratio is 14.7 and the output voltage of the O.sub.2 sensor rapidly changes in the vicinity of the air-fuel ratio of 14.7, control of the air-fuel ratio is performed by decreasing the fuel supply amount if the output voltage of the O.sub.2 sensor is high, while increasing it if the output voltage of the O.sub.2 sensor is low. As a result, under the air-fuel ratio control based on the output voltage of O.sub.2 sensor, the output voltage of the O.sub.2 sensor takes a substantially sinusoidal waveform in normal condition as shown in FIG. 33.
If the O.sub.2 sensor deteriorates, the variation of the output voltage for the change of the air-fuel ratio becomes slow (the frequency of the output voltage decreases) or the the output signal (difference between the maximum and minimum amplitude of values: P--P) becomes small, and thus the optimum air-fuel ratio cannot be maintained and the drivability of the vehicle lowers.
Since it was difficult to quantitatively measure such variations in frequency and amplitude by a device with simple construction, deterioration of the O.sub.2 sensor could not be detected by a prior art fault diagnostic system.
(2) Failure of EACV
The flow rate of the fuel gas flowing in EACV 170 is continuously controlled by the electric current supplied from EACV driver circuit 172 to the coil 179 of the solenoid 171. But, when the opening of bypass valve 178 cannot be controlled in proportion to the current supplied to coil 179 due to deterioration of the moving portion or jamming in EACV 170, since it is difficult to quantitatively measure this it has been difficult to detect these hindrances by a prior art fault diagnostic system.
(3) Failure of EGR
Inoperativeness of the valve plug 159 of EGR 150 can be detected by referring to the position signal from lift sensor 161, but inoperativeness of valve plug 159 is not always generated by failure of EGR 150 and it is also generated by failure of electromagnetic valve 182. In other words, even if valve plug 159 is inoperative, the real cause of that inoperativeness cannot be simply judged on the basis of the quantitative value detected by lift sensor 161, and thus it has been difficult to detect this by a prior fault diagnostic system.